The present invention relates to an apparatus for filtering and concentrating a suspension in general or a liquid which contains suspended solids therein and to a method therefor, and more particularly to a method of efficiently concentrating a suspension which has fine suspended particles therein.
In a concentrating technique by filtering a liquid that contains coarse suspended matter therein, in general, the filtrate is separated by using a mesh, a woven fabric or a nonwoven fabric as a filtering member. In the case of treating a liquid containing fine suspended matter therein, however, if use is made of a filtering member having a small openings therein to provide fine filtration, the filtering member is easily obstructed to lower the filtering efficiency.
When a liquid containing fine suspended matter therein is to be filtered and collected, an ultrafiltering method using a tubular filtering membrane which has tiny openings is used. Namely, the raw liquid or suspension to be treated permeates through the tubular filtering membrane while the raw liquid flows inside the tubular filtering membrane, and the filtrate that is filtered out of the tube is collected. In such a crossflow method, i.e., the method which performs the filtration by flowing the raw liquid in a direction in parallel with the filtering member, the problem of obstructing is not very serious when the raw liquid has a low concentration. However, as the raw liquid is concentrated to make the concentration of the suspended matter higher, the problem of obstructing appears conspicuous and the filtering efficiency drops rapidly. Moreover, when suspended matter has a high concentration, the filtering member must be washed frequently.
Therefore, it had not been considered advantageous to employ the crossflow-type filtering method as long as it is possible to employ an ordinary filtering method of the batch type or the semi-batch type, i.e., as long as it is allowed to employ the method in which the raw liquid is flowed against the filtering member to separate it into a filtrate and a cake, until the cake builds up to such an extent removal thereof from the filtering member is needed.